I am assuming that your project and applications are distinct entities.Also, by Project I assume that you are referring to an Eclipse/IntelliJ/Netbeans project
You would need to set the correct classpaths for the Project and your application separately.
If you are using the IDE to debug your project, your project would need to include the new JAR that you have downloaded. Each IDE has it's own mechanism for adding dependencies to the Project's classpath and hence you would want to refer to the documentation related to that IDE.
For your application you can launch it as java MyApp -cp "path to the old JAR". If you are packaging your application as a JAR, make sure you are packaging the older version of Guava.
Hope this helps.
Edit based on the discussion with the owner of the question
The query here is similar to this SO query. The right way to hence resolve this issue would be either install a Custom ClassLoader or use OSGi